Methods and systems for removing ice from surfaces

ABSTRACT

A coating ( 626 ) comprising a ferroelectric, lossy dielectric, ferromagnetic or semiconductor material is disposed near an object ( 620 ). AC current flows through an electrical conductor ( 624 ), creating an electromagnetic field. The coating ( 626 ) absorbs energy from the electromagnetic field, thereby generating heat, which melts snow and ice on the object ( 620 ).

[0001] This application is a continuation-in-part application ofcommonly-owned and copending PCT application PCT/US99/28330, filed Nov.30, 1999, and is based partly on U.S. provisional application Nos.60/122,463, filed on Mar. 1, 1999 and 60/131,082, filed on Apr. 26,1999, each of which is hereby incorporated by reference.

[0002] Funding for the invention was provided through DOD Award#DAAH04-95-1-0189 and NSF Award #MSS-9302797.

GOVERNMENT LICENSE RIGHTS

[0003] The U.S. Government has certain rights in this invention asprovided for by the terms of Grant #DAAH 04-95-1-0189 awarded by theArmy Research Office.

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] The invention relates to methods, systems and structures forremoving ice from surfaces.

[0006] 2. Statement of the Problem

[0007] Ice adhesion to certain surfaces causes many problems. Forexample, icing on power lines adds weight to the power lines causingthem to break. In addition to the costs of repair, the resulting poweroutages cause billions of dollars in direct and indirect economicdamage. The large surface areas of power lines exposed to icingconditions and the remoteness of many lines require de-icing systemsthat are both reliable and have low costs per unit distance.

[0008] Excessive ice accumulation on aircraft wings endangers the planeand its passengers. Ice on ship hulls creates navigational difficulties,the expenditure of additional power to navigate through water and ice,and certain unsafe conditions. The need to scrape ice that forms onautomobile windshields is regarded by most adults as a bothersome andrecurring chore; and any residual ice risks driver visibility andsafety.

[0009] Icing and ice adhesion also causes problems with helicopterblades, and with public roads. Billions of dollars are spent on ice andsnow removal and control. Ice also adheres to metals, plastics, glassesand ceramics, creating other day-to-day difficulties. In the prior art,methods for dealing with ice vary, though most techniques involve someform of scraping, melting or breaking. For example, the aircraftindustry utilizes a de-icing solution such as ethyl glycol to douseaircraft wings so as to melt the ice thereon. This process is bothcostly and environmentally hazardous; however, the risk to passengersafety warrants its use. Other aircraft utilize a rubber tube alignedalong the front of the aircraft wing, whereby the tube is periodicallyinflated to break any ice disposed thereon. Still other aircraftredirect jet engine heat onto the wing so as to melt the ice.

[0010] These prior art methods have limitations and difficulties. First,prop-propelled aircraft do not have jet engines. Secondly, rubber tubingon the front of aircraft wings is not aerodynamically efficient. Third,de-icing costs are extremely high, at $2500-$3500 per application; andit can be applied up to about ten times per day on some aircraft.

[0011] With respect to many types of objects, resistive DC heating ofice and snow is common. But, heating of some objects is technicallyimpractical. Also, large energy expenditures and complex heatingapparati often make heating too expensive.

SOLUTION

[0012] The invention provides systems and methods for removing orpreventing the formation of ice on power lines, airplane wings and otherobjects.

[0013] A system in accordance with the invention for preventing ice andsnow on a surface of an object contains an electrical conductor integralwith the surface. The conductor is configured to generate an alternatingelectromagnetic field in response to an AC current. A system alsoincludes a coating integral with both the surface and with theelectrical conductor. The coating is configured to generate heat inresponse to the alternating electromagnetic field. The coating containsa material selected from the group of materials consisting offerroelectric, lossy dielectric, semiconductor and ferromagneticmaterials. A conductor is “integral” with a surface if the surface iswithin an alternating electromagnetic field generated by an AC currentflowing in the conductor. A coating is “integral” with both a conductorand a surface if both the coating is within an alternatingelectromagnetic field generated by an AC current flowing in theconductor and if the heat generated by the coating prevents ice on thesurface. As a practical matter, both conductor and coating are commonlystructurally included in the object being protected from ice and snow,for example a power line or an airplane wing. When the heat-generatingcoating is included in the surface, or is in direct physical contactwith the surface, heat transfer between coating and surface is enhanced.Typically, the surface of a conductor itself is being protected; forexample, the surface of a conductive airplane wing may be protected bydisposing a coating in accordance with the invention on the wing surfaceand flowing AC current through the wing. The surface of a power line istypically an insulator casing enclosing the main conductors. Conductorsmay be formed on the surface of the object being protected by varioustechniques, including photolithography.

[0014] In many embodiments in accordance with the invention, forexample, in power lines, AC current is already present to generate thealternating electromagnetic field, causing heat in the coating. Inotherembodiments, a dedicated AC powersource may be used to provide ACcurrent; for example, in systems to de-ice airplane wings.

[0015] In a typical embodiment, the surface comprises the coating; forexample, a coating may adhere permanently to the surface of a powerline. In other instances, a coating may be embedded in the object beingprotected, below the surface exposed to icing; for example, a coating inaccordance with the invention may be formed as a layer enclosed withinin an airplane wing. Or a coating may be completely separate from theobject being protected, being disposed within an integral distance,either permanently or temporarily, to heat the surface of the object.

[0016] The coating may be a ferromagnetic material configured togenerate heat in response to an alternating magnetic field. Other typesof coating may be configured to generate heat in response to acapacitive AC current. In such embodiments, the AC current in theconductor creates an alternating electric field (“AEF”), that generatesa capacitive AC current in the coating. The capacitive AC current causesheating in the coating. In such embodiments, earth may function as asink for the capacitive AC current, or another power line may functionas a sink, or a special sink may be provided. The coating may comprisesemiconductor material configured to generate heat in response to acapacitive AC current. An example of such a semiconductor material isZnO. The coating may comprise ferroelectric material configured togenerate heat in response to a capacitive AC current. Typically, theferroelectric material has a dielectric constant that changes as afunction of temperature, the coating having a low dielectric constant ata temperature above freezing, and a high dielectric constant belowfreezing. For example, the ferroelectric material may have a Curietemperature, Tc, in the range of from 250° to 277° K. The coating maycomprise lossy dielectric material configured to generate heat inresponse to a capacitive AC current. The lossy dielectric material maybe chosen to have a dielectric loss maximum at an AC frequency in arange of from 40 to 500 Hz when relatively low-frequency AC current isused to prevent icing. On the other hand, the lossy dielectric materialmay have a dielectric loss maximum at an AC frequency in a range of from0.5 to 300 kHz when relatively high-frequency AC current is used toprevent ice. For example, if the coating has a dielectric loss maximumat 6 kHz, then the de-icing function can be turned “on” by switching theAC current from low frequency 60 Hz to 6 KHz. The coating thickness istypically selected to correspond to an amount of heat desired to begenerated by the coating. In a particularly simple embodiment, the lossydielectric material coating is ice itself. In embodiments applied topower lines, the power source typically provides AC current in a rangeof from 100 to 1000 kV.

[0017] An embodiment in accordance with the invention may include aconductive shell, the coating disposed between the electrical conductorand the conductive shell. An example is an aluminum conductive shellsurrounding the coating of a power line, thereby forming the outersurface of the power line. By electrically shorting the conductor andthe conductived shell when no de-icng is required, the capacitive ACcurrent in the coating is eliminated, no heat is generated by thecoating, and no energy is wasted. As with the conductor, the conductiveshell may be formed by photolithography. An embodiment typicallyincludes a switch for controlling the electrical connection that shortsconductor and conductive shell. An IGBT power semiconductor switch iswell suited. An embodiment typically comprises a control box derivingits power from the alternating electric field. The control box can beremotely controlled; for example, by a radio signal or by a carriersignal. The control box can also be controlled locally and autonomouslybased on input by a local sensor. For example, the local sensor mayinclude a temperature sensor or an impedance sensor for detecting ice. Atypical impedance sensor comprises a 100 kHz impedance sensor. In someembodiments, a control box may comprise a control box case capable ofserving as a capacitive antenna. An embodiment may include a number ofcontrol boxes, monitoring different sections of the system. For example,a plurality of control boxes may be spaced apart every 5 km or every 50km along a power line.

[0018] An embodiment may include a transformer to transform AC currenthaving a low-voltage to a higher voltage sufficient to generate heat ina coating. Such transformers, for example, may be located at appropriatedistance intervals along power lines.

[0019] Embodiments in which the coating is ice preferably include ameans for frequency-tuning the high-frequency AC current to match thestanding-wave effects of ice-dielectric heating and the skin-effectheating resulting from high-frequency current flow in a conductor. Anembodiment may also include a means for varying the high-frequency ACcurrent to change the heating pattern produced by standing wave effectsof ice-dielectric heating and skin-effect heating, thereby providingsufficient heat at all locations at various times to prevent icing.

[0020] In summary, AC current flows through an electrical conductor,creating an electromagnetic field. A coating absorbs the electromagneticenergy, generating heat. The heat from the coating heats the surface ofthe object being protected above the melting point of ice. The coatingmaterial may be a ferroelectric, a lossy dielectric, a semiconductor, ora ferromagnetic material. In some embodiments, the dielectric ormagnetic loss properties of the coating depend on a specifictemperature. These properties cause the absorption of electromagneticenergy and the resulting heating of the wires only when the ambienttemperature drops below the ice's melting point. In other embodiments,the absorption of energy depends on the frequency of the AC current. Asystem in accordance with the invention may also include a conductiveshell such that the coating material is between the conductor and theconductive shell. By electrically shorting the conductor and theconductive shell, the heating may be switched “off”, conserving energy.

[0021] In a particular variation, ice itself is utilized as a lossydielectric coating at high frequency, such as at 60 kHz. Further,skin-effect heating at high frequency may be combined with dielectricheating to melt ice and snow on power lines.

[0022] The invention is next described further in connection withpreferred embodiments, and it will become apparent that variousadditions, subtractions, and modifications can be made by those skilledin the art without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] A more complete understanding of the invention may be obtained byreference to the drawings, in which:

[0024]FIG. 1 depicts an embodiment of the invention suitable to reduceor remove ice from coated power lines;

[0025]FIG. 2 shows a cross-sectional view of a coated power linefabricated in accordance with the invention;

[0026]FIG. 3 shows an equivalent electric circuit of a power linewithout a coating;

[0027]FIG. 4 shows an equivalent electric circuit of an identical powerline with a coating;

[0028] FIGS. 5-7 show the results of calculations of maximum heatingpower, in units of watts per meter, as a function of voltage when usingdifferent thicknesses of a dielectric coating;

[0029]FIG. 8 shows the heat transfer from a 2.5 cm conductor as afunction of wind velocity with a ΔT of 10° C.;

[0030]FIG. 9 depicts heat transferred from a conductor plotted as afunction of conductor diameter with a wind velocity of 10 m/s;

[0031]FIG. 10 shows the temperature dependence of normalized heatingpower of a 1 mm thick coating of ZnO on a 1000 MW power line, where theice melting point is 273° K.;

[0032]FIG. 11 depicts a circuit diagram in which two resonance contoursare used to prevent a 6 kHz voltage from passing to a 60 Hz powersupply;

[0033]FIG. 12 depicts a power line de-icing system having a conductiveshell that can be shorted to the conductor;

[0034]FIG. 13 shows an improved embodiment of a power line with localcontrol of heating, having a coating, a conductive shell, a control box,an IGBT power switch, and local sensors;

[0035]FIG. 14 an embodiment having a transformer installed on power lineto provide energy to a coating;

[0036]FIG. 15 depicts a generalized structure and system in accordancewith the invention utilizing a dielectric, ferroelectric, ferromagneticor semiconductor coating to de-ice a non-active surface (i.e., a surfacewithout an internal AC electromagnetic field);

[0037]FIG. 16 depicts a cross-sectional view of a power line having anAC power supply to provide enenrgy to the dielectric, ferroelectric,ferromagnetic or semiconductor coating;

[0038]FIG. 17 depicts a cross-sectional view of a structure withspaced-apart electrodes;

[0039]FIG. 18 depicts a top view of an embodiment of FIG. 17;

[0040]FIG. 19 depicts a structure comprising a substrate surface onwhich spaced-aprt linear electrodes are disposed;

[0041]FIG. 20 depicts a de-icing system in which ice itself is used asthe lossy dielectric coating;

[0042]FIG. 21 shows a graph in which heating power (dissipated heat),Wh, in units of watts per meter, is plotted as a function of frequencyfor a layer of ice on a 5.1 cm diameter power-line cable, at a voltageof 30 kV;

[0043]FIG. 22 shows a graph in which heating power of ice-dielectricheating, skin- effect heating and their sum, in units of watts permeter, are plotted as functions of distance in meters, m, along a powerline;

[0044]FIG. 23 shows the results over a distance of 3000 m in a powerline when frequency of the AC current is tuned to balance ice-dielectricheating and skin effect heating to maximize total heating;

[0045]FIG. 24 depicts calculated percentage attenuation of heating powerin the improved embodiment of FIG. 23 over a distance of 50 km.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] The invention includes methods, systems and structures thatprevent ice and snow on surfaces of objects by using a coatingcongifured to absorb electromagnetic energy emanating from an ACcurrent. This absorption heats the coating, which heats the surface toprevent ice. Although embodiments in accordance with the invention aredescribed below principally with respect to power line de-icing, it isunderstood that the invention is useful in many types of applications.

[0047] De-Icing of Power Lines

[0048]FIG. 1 and FIG. 2 depict an embodiment in accordance with theinvention suitable to prevent or remove ice on power lines 100. FIG. 2shows a cross-sectional view 102 of power line 100 constructed inaccordance with the invention. A coating 106 having a thickness of “t”is applied over the line 104. As known in the art, a typical main powerline 104 carries power at 60 Hz, but with very high alternating electricfields, such as 4,000 volts/cm. The coating 106 generates heat in thepresence of an alternating electromagnetic field, such as generated bythe main power line 104. Specifically, it exhibits hysteresis thatgenerates heat over the AC cycle. Thus, heat is generated frompreviously unused power to melt or prevent ice on the power line. Thisembodiment utilizes the alternating electricmagnetic fields that alreadyexist due to the current flowing through the power line.

[0049] The coating 106 may be a ferroelectric material, as known in theart. Ferroelectric materials are essentially ceramics that exhibit avery high dielectric constant (e.g., 10,000 ) and very high dielectricloss (e.g., tan δ≅10) at certain conditions, and a relatively lowdielectric constant (3-5) and small dielectric loss at other conditions.One condition that can change the constant is temperature. Typically,the material is selected so that above freezing, the dielectric constantis low, and below freezing temperatures, the constant is high. Whenambient temperature drops below the freezing point, the coating isintensively heated by the alternating electric field (“AEF”) due to thehigh dielectric constant and dielectric loss.

[0050] More particularly, when a ferroelectric or lossy dielectricmaterial is placed in an AEF, the material is heated by the field due toa dielectric loss. The heating power per cubic meter is: $\begin{matrix}{W = {\frac{{\omega ɛɛ}_{o}}{4\pi}\tan \quad {\delta \left( \overset{\_}{E^{2}} \right)}}} & (1)\end{matrix}$

[0051] where ε is a relative dielectric permittivity (usually ε isapproximately 10⁴ for typical ferroelectrics), ε₀ is a dielectricpermittivity of free space (ε₀=8.85E−12 F/m), ω is an angular frequencyof the AC field (ω=2πf, where f is a usual frequency for the power line,e.g., 60Hz in conservative power lines), tan δ is the tangent ofdielectric loss, and ({overscore (E²)}) is the average of electric fieldsquared.

[0052] Ferroelectrics are characterized with very large values of ε andtan δ below the so-called Curie Temperature, T_(c), and small ε and tanδ above T_(c). Thus, the dielectric loss (or heating power of the ACelectric field) is very high below and close to T_(c); and it drops by alarge factor (e.g., 10⁶) above that temperature. This makesferroelectrics with T_(c) close to or just above the melting temperaturean optimum choice for a coating 10⁶ such as described above. Suchcoatings absorb the electric power when the outside temperatures dropbelow the melting point, T_(m), and are heated by the field to atemperature above T_(m) so that they again transform into usualinsulators (i.e., no longer absorbing the electric field in significantquantity).

[0053] Accordingly, when such coatings are placed in an AC field, theferroelectric material maintains a constant temperature which is closeto T_(c) and just above T_(m). This self-adjusting mechanism to preventicing is very economic: the maximum heating power per one meter of thepower line, or per m² at any surface to be protected, can be increasedor decreased by changing the coating thickness and/or by adding aneutral (notferroelectric) insulating paint or plastic to the coating.Examples of suitable ferroelectric materials according to the inventioninclude: TABLE 3 Ferroelectric materials Name Formula T_(c) (Kelvin)Rochelle salt NaKC₄H₄O₆4H₂O 255-297 Deuterated Rochelle salt NaKC₄H₂D₂O₆ 4H₂O 251-308 TGSe (NH₂CH₂COOH)₃ H2Se)₄ 295 Potassium tantalateKT_(a2/3) N_(b1/3) O₃ 271 niobate Anti momium nitrate NH₄NO₃ 255, 305Lead magnesium niobate Pb₃MgNb₂O₉ ˜273 (0° C.)

[0054] The thickness “t” is typically on the order of 0.5 mm to 10 mm,although other thicknesses can be applied depending upon coatingmaterials and desired heating. By changing the thickness, for example,temperatures at the surface 108 a can be increased by 1-10 degrees, ormore. The thickness “t” is chosen so that a desired amount of heat isgenerated (i.e., heat typically sufficient to melt ice and snow on thesurface 108 of the line 100 ).

[0055]FIG. 3 shows an equivalent electric circuit of a power linewithout a coating 106. Those skilled in the art understand the use ofthis configuration of resistors, capacitors, and inductors to representthe power line. Capacitance CL is an “interwire capacitance”. Without acoating, the capacitive AC current, I_(c), is not used to generate heat.FIG. 4 shows an equivalent electric circuit of an identical power linewith a dielectric or terroelectric coating 106. The coating 106 isrepresented in FIG. 4 by the resistances, R_(c) 422 and 426, and thecapacitances, C_(c) 424 and 428. In FIG. 4, a capacitive current flowingthrough the interwire capacitance C_(L) and the coating is representedby I′ 430. I′ 430 is less than I_(c) because of the added resistance andcapacitance of the coating 106. Thus, the power loss in the rest activeloads (R, R_(user)) decreases as a result of the heat dissipation in thecoating.

[0056] The heating power, W_(h), of a ferroelectric or lossy dielectriccoating on a cylindrical conductor is represented by the equation:$\begin{matrix}{W_{h} = \frac{V^{2}\omega^{2}R\quad C_{L}^{2}}{1 + {\omega^{2}{R^{2}\left( {C_{L} + C_{C}} \right)}^{2}}}} & (2)\end{matrix}$

[0057] where V is the voltage, ω is the angular frequency (2πf, R is theactive resistance (per meter) of the coating, C_(L) is an efficientinterwire capacitance, and C_(c) is the coating's capacitance (permeter). C_(L) may be calcualted using known techniques and includesinteractions between the conductor and various sinks of capacitive ACcurrent, for example other phase wires, ground wires and earth. Themaximum power occurs when: $\begin{matrix}{R = \frac{1}{\omega \left( {C_{I} + C_{C}} \right)}} & (3)\end{matrix}$

[0058] Combining Equations 25 and 26 will result in the maximum heatingpower, W_(max): $\begin{matrix}{W_{\max} = \frac{V^{2}\omega \quad C_{L}^{2}}{2\left( {C_{L} + C_{C}} \right)}} & (4)\end{matrix}$

[0059] When the coating reaches the condition for maximum power at thefrequency f₀=ω₀/2π, then the heating power at any other frequency f isshown in the following equation: $\begin{matrix}{W_{h} = \frac{2{W_{\max}\left( \frac{f}{f_{0}} \right)}^{2}}{1 + \left( \frac{f}{f_{0}} \right)^{2}}} & (5)\end{matrix}$

EXAMPLE 1

[0060] Exemplary heating power calculations were conducted forPb₃MgNb₂O₉. In this example, a middle range power line was consideredwith {square root}{square root over (v)}²=10 kV and with a wire diameterof 1 cm=2×radius. Accordingly, the electric field strength on the wiresurface is: $\begin{matrix}{E \approx \frac{V}{{\ln \left( \frac{L}{r} \right)}r} \approx {3\quad k\quad v\text{/}{cm}}} & (6)\end{matrix}$

[0061] where L is the distance between wires (L=1 m). Substitution asabove, i.e., {overscore (E)}²=3×10 V/m, ω=2π×60 Hz, ε=104 and tan δ=10,computes to W (at 1 mm, 60 Hz)=4.5×10⁵ watts/m³. A 1 mm thick film, forexample, thus generates 450 watt/m², which is more than sufficient fortypical melting of ice.

[0062] A frequency of 100 kHz at 300 kV heats a 1 mm thick coating ofPb₃MgNb₂O₉, at a rate 750 kWatt/m².

[0063] When applied to power lines, the maximum power that can bedissipated in the coating is limited by a capacitance C_(L) between thewires: $\begin{matrix}{W_{\max} = {\frac{\omega \quad C_{L}}{2} \cdot \overset{\_}{V^{2}}}} & (7)\end{matrix}$

[0064] For wires of 2 cm thickness, with 1 m distance between wires,C_(L)≅1.21E−11 F/m. For a power line at V=350 kV, W_(max)=300 Watt/m,which is sufficient energy to keep a 1 m long cable free of ice.

EXAMPLE 2

[0065] Using Equation (4), the maximum heating power, W_(max), in unitsof watts per meter, was calcualted as a function of voltage forconductors having different thicknesses of a dielectric coating on phase1 of a 3-phase power transmission system. The following variable valueswere used: radius, r, of the main conductor of the power line, 1.41 cm;radius of outside surface of coating was 1.41 cm plus the respectivecoating thickness; interwire distance, 7.26 m; phase to ground-wiredistance, 6.44 m; phase to earth distance, 20.24 m; ε₀, 8.85×10⁻¹²; δ,2.0; frequency, 60 Hz. FIG. 5 shows the heating power as a function ofvoltage for a dielectric coating of 10 mm thickness. FIG. 6 shows theheating power as a function of voltage for a dielectric coating of 5 mmthickness. FIG. 7 shows the heating power as a function of voltage for adielectric coating of 2 mm thickness. It is known that about 50 W/m isrequired to keep a power line free of ice.

[0066] In addition to thickness of dielectric coating, the heatingdissipation is also dependent on wind blowing on the power lines. Ifsignificant amounts of heat are removed from a power line by wind, thenan increase in heating power becomes necessary. The heat transfer from a2.54 cm conductor with a ΔT of 10° C. was calculated using conventionalmethods. In the graph of FIG. 8, heat transfer is plotted as a functionof the velocity of wind hitting the power line. The diameter of theconductor also will affect the heat transfer when wind is present. Inthe graph of FIG. 9, heat transfer is plotted as a function of thediameter of the conductor with a wind velocity of 10 m/s and ΔT of 10°C. In accordance with the invention, when the coating exhibits lowdielectric constant and loss (i.e., when the coating is above “freezing”or some other desired temperature), much less heat is generated by thecoating 306 and, thereby, much less energy is expended and lost by line302.

[0067] Those skilled in the art should appreciate that the surface ofobjects other than described herein can also be treated with thesecoatings. For example, applying such a coating to an airplane wing willalso provide melting capability by subjecting the coating to AC and,particularly, by increasing the voltage and frequency of the AC, asindicated in Equation (4) above.

[0068] In addition to ferroelectrics and dielectrics, almost anysemiconductor coating will provide similar effects. A semiconductor willabsorb the maximum energy from the external AC electric field when itsconductivity σ and dielectric permittivity ε satisfy the condition:$\begin{matrix}{\frac{{ɛɛ}_{0}}{\sigma} = {\tau_{\max} = \frac{1}{2\Pi \quad f}}} & (8)\end{matrix}$

[0069] where ε is the coating's dielectric constant, ε₀, is that of freespace, and f is the frequency of the AC field. As a result, thedielectric loss depends on the conductivity σ. To reach the maximumperformance of Equation (4), the coating dielectric conductivity σshould satisfy the condition:

[0070]   σ≈εε₀ω  (9)

[0071] where ε is the coating's dielectric constant, and ε₀ is that offree space. For a 60Hz line and ε≈10, σ≈3.4E-8 (ohm.m)⁻¹. Suchconductivity is very typical for many undoped semiconductors andlow-quality insulators. Thus, such a coating is not expensive (certainpaints qualify for these coatings). Moreover, temperature “tuning” canbe achieved due to a strong temperature dependence of conductivity ofsemiconductor materials (e.g., an exponential dependence).

[0072] One suitable material for semiconductive coatings is ZnO. FIG. 10shows the temperature dependence of normalized heating power of a 1 mmthick coating of ZnO on a 1000 MW power line, where the ice meltingpoint is 273° K. As suggested by the curve in FIG. 10, optimalconditions for the type of dielectric heating described above istypically satisfied only in a narrow temperature interval, e.g., −10°C.≦T≦10° C., where the coating will melt ice, otherwise consuming littlepower. Those skilled in the art understand that dopants could always beused to adjust the temperature interval.

[0073] Those skilled in the art should appreciate that theabove-described embodiment of ferroelectric and semiconductor coatingscan be self-regulating in keeping the coating temperature close to (orslightly above) the melting point. For example, if a ferroelectriccoating is overheated by the power line's electric field, itautomatically undergoes a phase transformation from the ferroelectric tothe normal state, at which point the coating stops absorbing theelectric field energy. By choosing a phase transition temperature,therefore, the coating temperature can be adjusted per user needs andper the environmental conditions of the local area.

[0074] In another embodiment, coating 306 of a power line is aferromagnetic material, as known in the art. In this case, the coatingabsorbs the energy of the magnetic field generated by a power line. Aferromagnetic coating with T_(C)=T_(M) melts ice in the same manner as aferroelectric material by converting the energy of a magnetic fieldgenerated by the AC current of the power line into heat.

[0075] Those skilled in the art should appreciate that the surface ofobjects other than described herein can also be treated with thesecoatings. For example, applying such a coating to an airplane wing willalso provide melting capability by subjecting the coating to AC and,particularly, by increasing the AC voltage and frequency, as indicatedby Equation (4) above.

[0076] A coating may be used containing a lossy dielectric materialhaving a maximum dielectric loss at higher frequencies, in a range offrom 0.5 to 300 kHZ. When an AC current in the conductor has a lowfrequency in a range of from 40 to 500 Hz, there is virtually no energydissipated as a result of dielectric heating. When the AC current has afrequency near its maximum dielectric loss frequency, then heatingoccurs. By switching between high and low AC frequencies, the heatingcan be switched “on” and “off”. The heating power for a given dielectriccoating material and set of operating conditions is calcualted usingEquations 2-5, above. The strong dependence of heating power ofdielectric coatings on the frequency shows why the power line is heatedwhen, for example, 6 kHz voltage is applied instead of 60Hz AC. Highfrequency AC current may be supplied using a separate AC power supply asa power source. Or, frequency multipliers as known in the art may beused to multiply the output of a low-frequency power supply to makehigh-frequency AC current. A sketch of a representative electricalcircuit diagram of an embodiment using hgih-frequency AC current todeice a power line in a 2-phase system is shown in FIG. 11. In FIG. 11,a power line supply 440 at 230 kV and 60 Hz is connected to a firstpower line 442 and a second power line 444. On the other end, a user 446is connected to power lines 442, 444. First power line 442 includescircuit unit 447 comprising inductance 448 in parallel with capacitance450. First power line 442 also includes series unit 453, comprisinginductance 454 in parallel with capacitance 456, which is in series withcircuit unit 447. A coating power supply 452 operates at 6 kHz with avoltage of 23 kV. Coating power supply 452 is connected to firstresonance contour 458. First resonance contour 458 is connected to powerline 442 between the two series circuit units 447 and 453. Coating powersupply 452 coupled to a second resonance contour 460, which is connectedto second power line 444. The two resonance contours, 458 and 460, areused to prevent a 6 kHz voltage from passing to the 60Hz power supply440 and user 446.

[0077]FIG. 12 shows an example of an embodiment containing a conductiveshell. FIG. 12 depicts a cross-sectional view of a power line 500. Thepower line 500 comprises cylindrically-shaped layers. The center of thepower line 500 is a steel core 502. Surrounding the steel core 502 aremain conductors 504, typically of aluminum. Outside the main conductorsis a coating 506, typically a lossy dielectric, ferroelectric orsemiconductive coating. The coating 506 is surrounded by an outerconductive shell 508, typically of aluminum. This embodiment providesfor a flexible de-icing technology in which a coating is heated withconventional 50-60 Hz electric field. The de-icing technique is fullycontrollable in that it can be swiched “on” or “off”. Thus, no electricpower is wasted when there are no icing or snow conditions. Withreference to the structure of FIG. 12, to switch the heating off, themain conductors 504 are electrically connected by switch 512 to anconductive shell 508, with the coating between them. This provides zeropotential difference across the ferroelectric, lossy-dielectric orsemiconductor coating 506 and, therefore, zero heating power. Theconductive shell 508 may be very thin (0.1 to 1 mm) and, therefore,inexpensive. The conductive shell 508 may comprise aluminum or anothermetal or any conductive or semiconductive material, for example,polyurethane impregnated with carbon. When connected with the ACconductor (most of the time), it increases total cable conduction.Switching “on” and “off” may be done with a radio-controlled remoteswitch. The power line company typically installs one such simple(low-voltage, low power compare to the line's voltage) switch aboutevery 100 km. Development of a lossy-dielectric coating then becomesinexpensive and simple because it must not be precisely “temperaturetuned”. Wider variety (and cheaper) materials can be used for thecoating. These features thus provide for an electric switch that enablesand disables heating of the power line selectively. Equivalentstructures and methods may be used for other objects, besides powerlines, to prevent or remove ice and snow. Such a system provides manyadvantages. First, the deicing can be fully controlled by the switch 512to deice the power lines on demand. Second, power levels can be variedto heat wires. Also, this embodiment may be applied to low voltage powerlines (below 100-345 kV), in addition to high voltage power lines. FIG.13 depicts a block diagram of de-icing system 520 including anembodiment having local, autonomous control and switching. Power line522 comprises main conductor 524, dielectric coating 526, and conductiveshell 528. When switch 532 is closed, conductive shell 528 and conductor524 are electrically shorted to each other. Preferably switch 532 is anIGBT power switch, which requires very little power. Control box 534includes control box casing 536, which serves as a capacitive antenna,collecting energy from the AEF and providing it to power supply 538.Power supply 538 supplies on the order of 0.1 watts power to control box534. Control box 534 also contains temperature sensor 540 and impedancesensor 542 for detecting ice. Impedance sensor 542 is preferably a 100kHz impedance sensor. Signals from temperature sensor 540 and impedancesensor 542 are processed by controller logic 544, which activates switch532 to open or close.

[0078]FIG. 14 depicts an embodiment including power line 550 connectedto a transformer 570. Power line 550 includes main conductor 552,coating 556, and conductive shell 558. Transformer 570 comprises aferroelectric core 572 covered by a winding 574. Winding 574 isconnected to conductive shell 558 and main conductor 552. Transformer570 functions in a conventional manner. For example, in FIG. 14, thevoltage drop along a 10 cm interval of main conductor 552 may typicallybe 1 millivolt. Transformer 570 typically transforms 60 Hz AC current atthis low voltage to a 60 Hz AC current having a voltage on the order of100 V to 200 V, sufficient to create an electromagnetic field capable ofcausing coating 506 to generate heat for de-icing along a distance onthe order of 200 to 400 meters.

[0079]FIG. 15 depicts a generalized structure and system 600 inaccordance with the invention utilizing a dielectric, ferroelectric orsemiconductor coating to de-ice a non-active surface (i.e., a surfaceswithout internal AC electric fields). In FIG. 15, a foil electrode 604is disposed on the surface 602 of a structure or object to be protectedfrom icing. A dielectric coating 606 is disposed on foil electrode 604.A foil electrode 608 is located on ferroelectric coating 606. Foilelectrodes 604, 608 provide for application of AC power to theferroelectric coating 606. The AC power derives from a standard AC powersupply 610. An ice detection system 612 (e.g., an detection system asdescribed with reference to FIG. 13), in circuit with the structure 600,preferably informs the power supply 610 of ice on the structure 600,whereinafter AC power is applied. The AC frequency and coating thicknessare chosen to generate heat at the desired quantities (e.g., so as tokeep icing from forming on an aircraft wing). This embodiment may alsobe applied to a power line. For example, FIG. 16 depicts power line 620,having steel core 622, main conductors 624, coating 626 and conductiveshell 628. A 60 Hz AC power supply 630 is located in series in serieswith switch 632, between conductors 624 and conductive shell 628.

[0080] The use of the AC power source provides many advantages. First,the deicing can be fully controlled by the switch 614 to deice the powerlines on demand. Second, power levels can be varied to heat wires. Also,this embodiment may be applied at low voltage (below 100-345 kV), inaddition to high voltage.

[0081] Embodiments in accordance with the invention also provide forspaced electrode configurations, as set forth below in FIGS. 17-19. FIG.17 depicts a cross-sectional view of a structure 700 with spaced-apartelectrodes 706. A substrate surface 702 is typically covered with acoating 704 and an outer conductive layer. Holes 708 through the outerconductive layer and insulating layer, down to the substrate surfaceform spaced apart electrodes 706. The space-to-space distance 712 istypically 10 to 100 μm. The total thickness of outer electrode layer 706is typically on the order of about 10 μm. FIG. 18 depicts a top view ofan embodiment in accordance with the invention as depicted in FIG. 17.Those skilled in the art should appreciate that different configurationsof the electrode spacing may be made. For example, in FIG. 19, astructure 720 comprises a substrate surface 722 on which linearelectrodes 724 are disposed. Preferably, the electrodes 724 are spacedapart by 10-50 μm, and each electrode has a width of 10-50 μm. Anexemplary fabrication method for making spaced-apart electrodes inaccordance with the invetnion includes: coating the surface withpolyurethane; applying a layer of photoresist; exposing with light theexposure region definging the electrode grid pattern (e.g., holes inFIG. 17, strips in FIG. 19); removing exposed regions to exposepolyurethane; applying graphite powder; heating to diffuse graphite intothe polyurethane. This method makes the electrodes durable andnon-corrosive. The resulting structure essentially contains an electrodegrid formed of plastic doped with carbon (a conductor), forming theexact pattern by photolithography.

[0082] In the embodiments described above, a dielectric coating wasdisposed on a power line, and either the interwire electric field or aspecifically applied AC voltage was used to heat the coating and,thereby, melt ice. In a further embodiment of the invention, depicted inFIG. 20, ice itself is used as the dielectric coating. FIG. 20 depictsthree typical power lines 802 in a 3-phase power transmission system,each comprising a steel core 804 surrounded by aluminum main conductors806 and covered by ice 810. Electric field lines 812 represent ahigh-frequency interwire electric field. Ice is a lossy dielectric witha maximum dielectric loss at so-called Debye frequency fD. When placedin an alternating electric field of that frequency and of sufficientstrength, ice melts. This is the same mechanism that uses an openlossy-dielectric coating (i.e., with no outer conductive layer), asdepicted in FIG. 2, but now with ice as the coating. FIG. 22 shows agraph in which heating power (dissipated heat), W_(h), in units of wattsper meter, is plotted as a function of frequency for a layer of ice on a5.1 cm diameter power-line cable, at a voltage of 30 kV. Under theseconditions, the required heating power of 50 to 150 watts per meter isachieved at a frequency of about 50 kHz. Thus, to reduce or eliminateicing of power lines, one applies an AC voltage of a high frequency tothe cables; for example, in the range of from 50 Hz to 150 Hz. Whenthere is no ice, there is no power consumption. This provides aninexpensive and simple solution to the problem of icing. When iceappears on the cables, the system works as a dielectric coating heatedby the AC electric field, melting the ice. Water on the cables does notabsorb AC power because water has a dielectric-loss maximum in amicrowave frequency range. The same principle works for refrigeratorsand for airplanes. The ice-dielectric heating may be switched “on” bysupplying high-frequency AC current in the main conductors when ice ispresent; it may be turned “off” by using low-frequency AC, for example60 Hz.

[0083] In a further embodiment in accordance with the invention,skin-effect heating is used to melt ice on a long-distance power line. Amagnetic field pushes electrical current lines towards the surface of aconductor. In a case of high-frequency current flow in aluminum at 60kHz, for example, the current flows in the outer 0.35 mm of theconductor. For a power line with a diameter of 2.5 cm, this currentcrowding increases the resistance by a factor of approximately 20. With221 amps of current, this results in a maximum heating power ofapproximately 50 W/m. Unlike ice-dielectric heating, skin-effect heatingoccurs even when no ice is present. Thus, energy dissipation ofskin-effect heating can only be switched “off” by using low-frequency ACcurrent.

[0084] A further embodiment of the invention combines high-frequency(“HF”) ice dielectric heating and the HF skin-effect heating. Both icedielectric loss and the skin effect are subject to standing wavephenomena, as depicted in FIG. 22. By using known frequency-tuningmeans, however, the peaks and valleys of heating from the two effectsmay be made to complement each other. This effects of this embodimentare depicted in the graph of FIG. 23, in which heating power, W_(h), inunits of watts per meter, is plotted as a function of distance inmeters, m, from a power source. FIG. 23 shows that the total heat effectis relatively constant at about 50 W/m over a distance of 3000 m. In animproved embodiment, the frequency of the current through the power lineis varied to balance the heating effects at various spatial locations ofthe conductor. FIG. 24 depicts the calculated percentage attenuation ofheating effects of the improved embodiment of FIG. 20 over a distance of50 km. The data of FIG. 24 indicate that a 100 km power line could beheated and de-iced using a single driver located at the center. Forexample, the power source for a 50 km line should possess about 3.25 MWat 60 kHz. With a total heating power, W_(h), of 50 W/m, and aconvective loss of 25 W/m, leaving a net heating power of 25 W/m,calculations indicate that 0.5 cm of ice may be removed from the threephases of a 3-phase transmission system in about 3 hours, by switchingthe heating about every 10-20 minutes between power lines.

[0085] The various embodiments in accordance with the invention providerelatively simple, reliable and inexpensive systems and methods forpreventing and removing ice on the surface of an object. Although theembodiments have been described principally with regard to power linede-icing, the structures and methods herein described are applicable tomany other types of objects. Since certain changes may be made in theabove apparatus and methods without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing be interpreted asillustrative and not in a limiting sense.

1. A system of preventing ice and snow on a surface of an object,comprising: an electrical conductor integral with said surface, saidconductor configured to generate an alternating electromagnetic field inresponse to an AC current; a coating integral with said surface and withsaid electrical conductor, said coating configured to generate heat inresponse to said alternating electromagnetic field, said coatingcomprising a material selected from the group of materials consisting offerroelectric, lossy dielectric, semiconductor and ferromagneticmaterials.
 2. A system as in claim 1, further comprising an AC powersource capable of providing AC current.
 3. A system as in claim 2,wherein said AC power source is capable of providing an an AC currenthaving a voltage in a range of from 100 to 1000 kV.
 4. A system as inclaim 1, wherein said object comprises said electrical conductor.
 5. Asystem as in claim 1, wherein said electrical conductor is formed byphotolithography.
 6. A system as in claim 5, comprising a plurality ofspaced apart electrical conductors formed by photolithography.
 7. Asystem as in claim 5, comprising a plurality of linear electricalconductors formed by photolithography.
 8. A system as in claim 1,wherein said surface comprises said coating.
 9. A system as in claim 1,further comprising a sink for capacitive AC current.
 10. A system as inclaim 1, wherein said coating is a ferromagnetic material configured togenerate heat in response to an alternating magnetic field.
 11. A systemas in claim 1, wherein said coating comprises semiconductor materialconfigured to generate heat in response to a capacitive AC current. 12.A system as in claim 11 wherein said semiconductor material comprisesZnO.
 13. A system as in claim 1, wherein said coating comprisesferroelectric material configured to generate heat in response to acapacitive AC current.
 14. A system as in claim 13, wherein saidferroelectric material has a dielectric constant which changes as afunction of temperature, said coating having a low dielectric constantat a temperature above freezing, and a high dielectric constant belowfreezing.
 15. A system as in claim 13, wherein said ferroelectricmaterial has a Curie Temperature, Tc, in the range of from 250° to 277°K.
 16. A system as in claim 1, wherein said coating comprises lossydielectric material configured to generate heat in response to acapacitive AC current.
 17. A system as in claim 16 wherein said lossydielectric material has a dielectric loss maximum at an AC frequency ina range of from 40 to 500 Hz.
 18. A system as in claim 16 wherein saidlossy dielectric material has a dielectric loss maximum at an ACfrequency in a range of from 0.5 to 300 kHz.
 19. A system as in claim 16wherein said lossy dielectric material is ice.
 20. A system as in claim1 wherein said coating has a thickness, said thickness selected tocorrespond to an amount of heat desired to be generated by said coating.21. A system as in claim 1, further comprising a conductive shell, saidcoating disposed between said electrical conductor and said conductiveshell.
 22. A system as in claim 21, wherein said conductive shellcomprises aluminum.
 23. A system as in claim 21, wherein said conductiveshell is formed by photolithography.
 24. A system as in claim 23,comprising a plurality of spaced apart conductive shells formed byphotolithography.
 25. A system as in claim 23, comprising a plurality oflinear conductive shells formed by photolithography.
 26. A system as inclaim 21, wherein said electrical conductor and said conductive shellcan be electrically shorted by an electrical connection.
 27. A system asin claim 26, further comprising a switch for controlling said electricalconnection.
 28. A system as in claim 27, wherein said switch comprisesan IGBT power semiconductor switch.
 29. A system as in claim 27, furthercomprising a control box, said control box capable of deriving its powerfrom an alternating electric field.
 30. A system as in claim 29, whereinsaid control box can be remotely controlled.
 31. A system as in claim30, wherein said control box can be remotely controlled by a radiosignal.
 32. A system as in claim 30, wherein said control box can beremotely controlled by a carrier signal.
 33. A system as in claim 29,wherein said control box can be controlled locally and autonomouslybased on input by a local sensor.
 34. A system as in claim 33, whereinsaid local sensor includes a temperature sensor.
 35. A system as inclaim 33, wherein said local sensor includes an impedance sensor.
 36. Asystem as in claim 35, wherein said impedance sensor comprises a 100 kHzimpedance sensor.
 37. A system as in claim 29, wherein said control boxcomprises a control box case, said control box case capable of servingas a capacitive antenna.
 38. A system as in claim 29, comprising aplurality of spaced apart control boxes.
 39. A system as in claim 2further comprising a transformer, said transformer increasing thevoltage of low-voltage AC current.
 40. A system as in claim 2 whereinsaid AC power source is capable of providing low-frequency AC current ina range of from 40 to 500 Hz.
 41. A system as in claim 2 wherein said ACpower source is capable of providing high-frequency AC current in arange of from 0.5 to 300 kHz.
 42. A system as in claim 2 comprising alow-frequency AC power source capable of providing low-frequency ACcurrent in a range of from 40 to 500 Hz and an AC power source capableof providing high-frequency AC current in a range of from 0.5 to 300kHz.
 43. A system as in claim 42 wherein AC current is switchablebetween said low-frequency AC current and said high-frequency ACcurrent.
 44. A system as in claim 1 further comprising a high-frequencyimpedance measuring device to detect ice.
 45. A system as in claim 1,wherein said coating is ice, and further comprising a means forfrequency-tuning said high-frequency AC current to match ice dielectricheating and skin-effect heating.
 46. A method as in claim 1, whereinsaid coating is ice, and further comprising a means for varying saidhigh-frequency AC current to change the heating pattern produced bystanding wave effects of ice-dielectric heating and skin-effect heating.47. A system as in claim 1 wherein said object is an electrical powerline, said electrical conductor is a main conductor of said power line,and said coating surrounds said main conductor.
 48. A system as in claim47 further comprising a plurality of electrical power lines.
 49. Asystem as in claim 47 wherein said main conductor comprises aluminum.50. A system as in claim 47, further comprising a conductive shell. 51.A system as in claim 50, wherein said conductive shell comprisesaluminum.
 52. A system as in claim 47 wherein said coating is ice andsaid AC power source is capable of providing high-frequency AC currentin a range of from 0.5 to 300 kHz.
 53. A system as in claim 52 furthercomprising a low-frequency AC power source capable of providinglow-frequency AC current in a range of from 40 to 500 Hz.
 54. A systemas in claim 53 wherein AC current is switchable between saidlow-frequency AC current and said high-frequency AC current.
 55. Asystem of preventing ice and snow on a surface of an object, comprising:an electrical conductor integral with said surface, said conductorconfigured to generate an alternating electromagnetic field in responseto an AC current.
 56. A system as in claim 55, further comprising an ACpower source capable of providing high-frequency AC current having afrequency in a range of from 0.5 to 300 kHz.
 57. A system as in claim55, further comprising a coating of ice integral with said electricalconductor.
 58. A system as in claim 55 further comprising alow-frequency AC power source capable of providing low-frequency ACcurrent in a range of from 40 to 500 Hz and a means for switching offsaid high-frequency AC current.
 59. A method of preventing ice and snowon a surface of an object, comprising: providing an electrical conductorintegral with said surface, said conductor configured to generate analternating electromagnetic field in response to an AC current;providing a coating integral with said surface and with said electricalconductor, said coating configured to generate heat in response to analternating electromagnetic field, said coating comprising a materialselected from the group of materials consisting of ferroelectric, lossydielectric, semiconductor and ferromagnetic materials; flowing an ACcurrent in said electrical conductor to produce an alternatingelectromagnetic field encompassing said coating.
 60. A method as inclaim 59, wherein said AC current has a voltage in a range of from 100to 1000 kV.
 61. A method as in claim 59, wherein said object comprisessaid electrical conductor.
 62. A method as in claim 59, wherein saidelectrical conductor is formed by photolithography.
 63. A method as inclaim 59, comprising a plurality of spaced apart electrical conductorsformed by photolithography.
 64. A method as in claim 59, comprising aplurality of linear electrical conductors formed by photolithography.65. A method as in claim 59, wherein said surface comprises saidcoating.
 66. A method as in claim 59, wherein said alternatingelectromagnetic field comprises an alternating magnetic field and saidcoating is a ferromagnetic material configured to generate heat inresponse to said alternating magnetic field.
 67. A method as in claim59, wherein said alternating electromagntic field generates a capacitiveAC current in said coating and said coating comprises semiconductormaterial configured to generate heat in response to said capacitive ACcurrent.
 68. A method as in claim 67, wherein said semiconductormaterial is ZnO.
 69. A method as in claim 59, wherein said alternatingelectromagnetic field generates a capacitive AC current in said coatingand said coating comprises ferroelectric material configured to generateheat in response to said capacitive AC current.
 70. A method as in claim69, wherein said ferroelectric material has a dielectric constant whichchanges as a function of temperature, said coating having a lowdielectric constant at a temperature above freezing, and a highdielectric constant below freezing.
 71. A method as in claim 69, whereinsaid ferroelectric material has a Curie Temperature, Tc, in the range offrom 250° to 277° K.
 72. A method as in claim 59, wherein saidalternating electromagnetic field generates a capacitive AC current insaid coating and said coating comprises lossy dielectric materialconfigured to generate heat in response to said capacitive AC current.73. A method as in claim 72, wherein said lossy dielectric material hasa dielectric loss maximum at an AC frequency in a range of from 40 to500 Hz.
 74. A method as in claim 72, wherein said lossy dielectricmaterial has a dielectric loss maximum at an AC frequency in a range offrom 0.5 to 300 kHz.
 75. A method as in claim 72, wherein said lossydielectric material is ice.
 76. A method as in claim 59, wherein saidcoating has a thickness, said thickness selected to correspond to anamount of heat desired to be generated by said coating.
 77. A method asin claim 59, further comprising providing a conductive shell, saidcoating disposed between said electrical conductor and said conductiveshell.
 78. A method as in claim 77, wherein said conductive shellcomprises aluminum.
 79. A method as in claim 77, further comprisingelectrically shorting said electrical conductor and said conductiveshell.
 80. A method as in claim 77, further comprising operating aswitch for controlling said shorting.
 81. A method as in claim 80,wherein said switch comprises an IGBT power semiconductor switch.
 82. Amethod as in claim 80, further comprising providing a control box, saidcontrol box capable of deriving its power from an alternating electricfield.
 83. A method as in claim 82, further comprising remotelycontrolling said control box.
 84. A method as in claim 83, comprisingremotely controlling said control box by a radio signal.
 85. A method asin claim 83, comprising remotely controlling said control box by acarrier signal.
 86. A method as in claim 82, comprising controlling saidcontrol box using input by a local sensor.
 87. A method as in claim 86,wherein said local sensor includes a temperature sensor.
 88. A method asin claim 86, wherein said local sensor includes an impedance sensor. 89.A method as in claim 88, wherein said impedance sensor comprises a 100kHz impedance sensor.
 90. A method as in claim 82, wherein said controlbox comprises a control box case, said control box case serving as acapacitive antenna.
 91. A method as in claim 59, comprising flowing alow-frequency AC current in said electrical conductor having a frequencyin a range of from 40 to 500 Hz.
 92. A method as in claim 59 comprisingflowing a high-frequency AC current in said electrical conductor havinga frequency in a range of from 0.5 to 300 kHz.
 93. A method as in claim59 comprising flowing a low-frequency AC current in a range of from 40to 500 Hz and then flowing a high-frequency AC current in a range offrom 0.5 to 300 kHz.
 94. A method as in claim 93 comprising operating atransformer for transforming said low-frequency AC current into saidhigh-frequency AC current.
 95. A method as in claim 93 comprisingswitching between said low-frequency AC current and said high-frequencyAC current.
 96. A method as in claim 59, wherein said object is anelectrical power line, said electrical conductor is a main conductor ofsaid power line, and said coating surrounds said main conductor.
 97. Amethod as in claim 96, wherein said main conductor comprises aluminum.98. A method as in claim 96, further comprising providing a conductiveshell, said coating disposed between said main conductor and saidconductive shell.
 99. A method as in claim 98, wherein said conductiveshell comprises aluminum.
 100. A method as in claim 98, furthercomprising electrically shorting said main conductor and said conductiveshell.
 101. A method as in claim 100, further comprising operating aswitch for controlling said shorting.
 102. A method as in claim 96,further comprising conducting high-frequency impedance measurements todetect ice.
 103. A method as in claim 96, wherein said coating is ice,and further comprising frequency-tuning said high-frequency AC currentto match ice dielectric heating and skin-effect heating.
 104. A methodas in claim 96, wherein said coating is ice, and further comprisingvarying said high-frequency AC current to change the heating patternproduced by standing wave effects.
 105. A method of preventing ice andsnow on a surface of an object, comprising: providing an electricalconductor integral with said surface, said conductor configured togenerate an alternating electromagnetic field in response to an ACcurrent; flowing a high-frequency AC current having a frequency in arange of from 0.5 to 300 kHz.
 106. A method as in claim 105, furthercomprising a coating of ice on said surface.
 107. A method as in claim105, wherein said object is a power line.
 108. A method as in claim 105,further comprising switching off said high-frequency AC current.